Spatial scale of non-target effects of cotton insecticides

Authors: BORDINI, ISADORA - University Of Arizona; Naranjo, Steven; FOURNIER, A - University Of Arizona; ELLSWORTH, P - University Of Arizona

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/8/2023
Publication Date: 5/10/2023
Citation: Bordini, I., Naranjo, S.E., Fournier, A., Ellsworth, P. 2023. Spatial scale of non-target effects of cotton insecticides. PLOS ONE. 18(5). Article e0272831. https://doi.org/10.1371/journal.pone.0272831.
DOI: https://doi.org/10.1371/journal.pone.0272831

Interpretive Summary: Any experimental work conducted in the field needs to consider plot size, especially those that include arthropods that can move within the environment. Inter-plot movement can interfere with the researcher’s ability to discern treatment effects within the experiment. Thus, plots need to be as large as practically possible relative to the organisms being studies. We conducted a 2-year replicated field experiment to determine the optimal plot size for non-target arthropod studies in the Arizona cotton system. We examined plot sizes of 144 m2, 324 m2 and 576 m2 and compared them using three insecticidal treatments; an untreated check, a positive control insecticide with known negative effects on the arthropod community and a selective insecticide. We compared plot sizes using several metrics, including arthropod community structure, community diversity, individual key species abundance, effect sizes, and biological control function of arthropod taxa with a wide range of mobility, including Collops spp., Orius tristicolor, Geocoris spp., Misumenops celer, Drapetis nr. divergens and Chrysoperla carnea. We found no effect of plot size for any metric and concluded that the smallest plot size of 144 m2 is sufficiently large to measure insecticidal effects on non-target arthropods in Arizona cotton. These results will be useful for other researchers in cotton and other crop, and for industry and regulatory agencies that frequently use field plots to assess environmental effects of new insecticides and other pest control technologies.

Technical Abstract: Plot size is of practical importance for any experiments involving mobile species. Plots need to be as large as practically possible for experiments to be applicable to commercial scales. Species naturally range freely over large agricultural areas, thus certain patterns and interactions might not be detected if a plot is too small. We conducted a 2-year replicated field experiment to estimate the optimal plot size for non-target arthropod studies in the Arizona cotton system. Experimental treatments consisted of plot sizes and insecticides in a full factorial. We established three plot sizes that measured 144 m2, 324 m2 and 576 m2. For insecticides, we established an untreated check, a positive control insecticide with known negative effects on the arthropod community and a selective insecticide. We investigated how plot size impacts the estimation of treatment effects relative to community structure (27 taxa), community diversity, individual species abundance, effect sizes, and biological control function of arthropod taxa with a wide range of mobility, including Collops spp., Orius tristicolor, Geocoris spp., Misumenops celer, Drapetis nr. divergens and Chrysoperla carnea. There was no effect of plot size. We concluded that the smallest plot size of 144 m2 is sufficiently large to measure insecticidal effects on non-target arthropods in Arizona cotton. 144 m2 plots supported similar results for all parameters compared to larger plots, and we were able to clearly measure the known destructive effects of the positive control in this plot size.